Method of reducing registration error in exposure step of semiconductor device

ABSTRACT

A method of reducing a registration error is provided in which the registration error can be uniformly distributed even if an amount of displacement is larger than others at only one of a plurality of measuring points. According to the method, amounts of displacement are measured first at a plurality of measuring points, then one half the sum of the maximum value and the minimum value of the measured amounts of displacement is calculated to obtain a correction value. The correction value is fed back to an exposure apparatus as a correction value for an exposure condition setting file within the exposure apparatus used in an exposure step. The registration error can be distributed uniformly even if amounts of displacement at a plurality of measuring points are considerably different from each other.

This application is a Divisional of application Ser. No. 08/653,146filed May 24, 1996, U.S. Pat. No. 5,731,113.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of reducing a registration error and,more particularly to a method of reducing a registration error in anexposure step of a process for manufacturing a semiconductor device.

2. Description of the Background Art

As a pattern of a semiconductor device is miniaturized, the demand forregistration accuracy in a photolithography step is increasing. Forexample, the design rule which refers to the minimum size in the designof a 64MDRAM (Dynamic Random Access Memory) is approximately 0.30-0.35μm. The registration accuracy required for this design rule is 0.08 μmor less.

FIG. 3 is a perspective view illustrating a conventional exposureapparatus. Referring to FIG. 3, the conventional exposure apparatus isprovided with a wafer 101 on a wafer stage 100 which is movable in theX-Y directions. A demagnification projection lens 103 is provided abovewafer 101, and a reticle 104 with a patterned layer is placed abovedemagnification projection lens 103. Using the exposure apparatus ofabove described structure, a pattern corresponding to the pattern imageof reticle 104 is formed on the surface of wafer 101. A shot 102 havingone pattern which corresponds to the pattern image of reticle 104 isformed on wafer 101 by an exposure, and a plurality of shots 102 areformed on wafer 101 by repeating the exposure.

An alignment at the time of exposure by the conventional exposureapparatus is performed by moving wafer stage 100 according to an arrayof shots 102 in the layer pattern (not shown) formed on wafer 101 in theprevious step. A general method conventionally used for recognizing thearray of shots 102 is EGA (Enhanced Global Alignment). According to thisEGA method, the state of placement of some of those shots 102 on wafer101 is determined by measuring the positions of the marks forregistration (alignment marks) provided in the shots 102. The arraystate of all the remaining shots 102 is accordingly examined. Based onthe result, an alignment exposure is executed.

Conventionally, after an exposure according to EGA, an amount ofdisplacement of shot 102 was measured by an alignment inspection device.The amount of displacement means an amount of shift (offset) in the Xand Y directions as shown in FIG. 4. For measuring the amount ofdisplacement, marks for inspection of misalignment (not shown) are firstarranged in four corners of shot 102. Then, a relative amount ofdisplacement between the first misalignment inspection marks of thefirst layer pattern formed in the previous step and the secondmisalignment inspection marks of the second resist pattern formed on thesecond layer on the first layer pattern is image-processed by a CCDcamera, whereby the amount of displacement is calculated. An averagevalue of the amounts of displacement at the four corners of shot 102 wasconventionally used as an amount of displacement of shot 102.

If all the shots 102 are determined to be uniformly shifted in the samedirection as a result of above inspection by the conventional alignmentinspection device, it is understood that a correction for the offsetamount was necessary beforehand in the exposure step according to EGA.That is, EGA requires a correction of the amount measured in EGA methodin accordance with the positions of the marks for registration(alignment marks) or the production error of a reticle. Feeding backthis correction value into an exposure condition setting file within theexposure apparatus, an offset error could be zero when the same exposurestep as above will be next performed.

With reference to FIG. 5, conventional ways of calculating and feedingback a correction value will be described in the following.

Referring to FIG. 5, a first layer is formed on a semiconductorsubstrate (not shown) in S1 (Step 1). In S2, a first resist is appliedto the first layer. Next, the applied first resist is exposed by a firstexposure apparatus in S3. In S4, a first resist pattern is then formedby developing the exposed first resist. In S5, the first layer ispatterned by etching using the first resist pattern thereon as a mask,thereby forming a first layer pattern. Next, a second layer is formed onthe first layer pattern in S6. A second resist is then applied to thesecond layer in S7. In S8, the second resist is exposed by a secondexposure apparatus. In S9, a second resist pattern is formed bydeveloping the exposed second resist.

Subsequently, amounts of displacement between the first layer patternand the second resist pattern are measured at four points (in fourcorners of a shot) in S10. A determination is made in S11 whether atleast one of the four measured amounts of displacement is a prescribedvalue or more. If all the amounts of displacement are less than theprescribed value, next in S12, a second layer pattern is formed byetching the second layer using the second resist pattern thereon as amask.

On the other hand, if at least one of the amounts of displacement isdetermined to be the prescribed value or more, a correction value iscalculated and a second resist is formed again. The correction value isdetermined by firstly calculating an average value of the four measuredamounts of displacement as shown in S15. Secondly the determinedcorrection value is fed back to the second exposure apparatus in S8.After above steps, a second resist is formed again. According to there-formation step of the second resist, the second resist pattern isfirst removed in S14, then the new second resist is applied again to thesecond layer in S7.

In the conventional method described above, if an amount of displacementbetween the first layer pattern and the second resist pattern is aprescribed value or more, a correction value is calculated and the valueis fed back to the second exposure apparatus, then the defective secondresist pattern is removed and a second resist pattern is newly formed byapplying a new second resist to the second layer.

According to the conventional method for exposing a plurality of layers,in some cases an exposure apparatus (first exposure apparatus) employedfor a pattern formation (first resist pattern formation) in the previousstep is different from an exposure apparatus (second exposure apparatus)employed for a pattern formation (second resist pattern formation) inthe following step.

Under above described condition in which a plurality of differentexposure apparatuses (steppers) are alternately used, shapes of shotsproduced by respective exposure apparatuses may be different from eachother due to the difference of lens distortion or the like betweenrespective exposure apparatuses. FIG. 6 is a schematic illustration ofan exemplary shape of a shot when different exposure apparatuses areemployed for the first layer and the second layer. Referring to FIG. 6,the shape of the shot of a first layer pattern 1 is approximatelyrectangular, while the shape of the shot of a second resist pattern 2 istrapezoidal. As a result, when amounts of displacement between the firstlayer pattern 1 and the second resist pattern 2 are measured at fourmeasuring points 3 according to S10 of FIG. 5, only an amount ofdisplacement "a" in the X direction at the measuring point in the upperright corner is measured. This result also occurs even when a singleexposure apparatus is employed, due to the change of an exposureillumination condition.

According to the conventional method of calculating a correction valueshown in S15 of FIG. 5, an average value (0+0+0+a)×1/4=a/4 of the fouramounts of displacement at the four corners of the shot is computed as acorrection value. The correction value is fed back to the secondexposure apparatus as an offset correction value upon the next exposure.The result of an alignment inspection after the exposure by the secondexposure apparatus with the correction value fed back is as shown inFIG. 7. An average value of the amounts of displacement at the fourcorners of the shot is (-a/4-a/4-a/4+3a/4)×1/4=0. However, amounts ofdisplacement at respective measuring points in the shot are -a/4, -a/4,-a/4, +3a/4, which means that there is left a point with an amount ofdisplacement of at most 3a/4 in absolute value. If any of the amounts ofdisplacement has such a larger value, various problems such as anelectrical short, caused, for example, by a shift of a pattern of acontact hole, will occur. Such problems have become serious with theincreasing miniaturization of a semiconductor pattern.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method of reducing aregistration error which can be applicable even if requirement forregistration accuracy is severe.

Another object of the invention is to provide a method of reducing aregistration error which can reduce the registration error if one of aplurality of measuring points has a large amount of displacement.

A method of reducing a registration error according to an aspect of theinvention is the one used in an exposure step of a process ofmanufacturing a semiconductor device. According to this method, afteramounts of displacement are measured at a plurality of measuring points,one half the sum of the maximum and the minimum values of those measuredamounts of displacement is calculated to obtain a correction value. Thecorrection value is employed as a correction value for an exposureapparatus used in the exposure step. Thus, this method allows theeffectively reduced registration error even if an amount of displacementat one of a plurality of measuring points is noticeably larger thanothers, since the larger amount can be distributed over the entire shot.

In a method of reducing a registration error according to another aspectof the invention, a first layer is formed on a first semiconductorsubstrate. A first resist is formed on the first layer. A first resistpattern with a plurality of first misalignment inspection marks isprovided by exposing a prescribed area of the first resist using a firstexposure apparatus. A first layer pattern with the first misalignmentinspection marks is formed by patterning the first layer using the firstresist pattern as a mask. A second layer is formed on the first layerpattern. A second resist is formed on the second layer. A second resistpattern with a plurality of second misalignment inspection markscorresponding to the plurality of first misalignment inspection marks isprovided by exposing a prescribed area of the second resist utilizing asecond exposure apparatus. Amounts of displacement between the firstmisalignment inspection marks of the first layer pattern and the secondmisalignment inspection marks of the second resist pattern are measuredat a plurality of measuring points. A correction value is obtained bycalculating one half the sum of the maximum and the minimum values ofthe amounts of displacement at the plurality of measuring points. Thecorrection value is then fed back to the second exposure apparatus.According to the method described above, an amount of displacement ismore effectively distributed over the entire shot when the amount atonly one of the measuring points is extremely large, compared with thecase in which an average value of the amounts of displacement at aplurality of measuring points is used as a correction value, since acorrection value is obtained by calculating one half the sum of themaximum and the minimum values of the amounts of displacement at aplurality of measuring points and the correction value is fed back tothe second exposure apparatus. Thus registration error can beeffectively reduced. In addition to those steps according to theaforementioned another aspect of the invention, steps of successivelyforming a first layer pattern on a second semiconductor substrate thenforming a second resist thereon and a step of forming a second resistpattern by exposing the second resist using the second exposureapparatus with the correction value fed back may be provided.

In a method of reducing a registration error according to still anotheraspect of the invention, a first layer is formed on a firstsemiconductor substrate. A first resist is formed on the first layer. Afirst resist pattern with a plurality of first misalignment inspectionmarks is provided by exposing a prescribed area of the first resistusing a first exposure apparatus. A first layer pattern with the firstmisalignment inspection marks is provided by patterning the first layerusing the first resist pattern as a mask. A second layer is formed onthe first layer pattern, then a second resist is formed on the secondlayer. A second resist pattern with a plurality of second misalignmentinspection marks corresponding to the plurality of first misalignmentinspection marks is provided by exposing a prescribed area of the secondresist using a second exposure apparatus. Amounts of displacementbetween the first misalignment inspection marks of the first layerpattern and the second ones of the second resist pattern are measured ata plurality of measuring points. It is determined whether at least oneof the absolute values of the measurements at the plurality of measuringpoints is a prescribed value or more. If a measurement is determined tobe equal or larger than a prescribed value, a correction value isobtained by calculating one half the sum of the maximum and the minimumvalues of the measurements, then the correction value is fed back to thesecond exposure apparatus. According to this method, a correction valuecan be calculated such that a large amount of displacement at one of aplurality of measuring points would be distributed over the entire shot,since it is determined whether at least one of absolute values ofmeasurements of the amounts of displacement at a plurality of measuringpoints is less than a prescribed value, then one half the sum of themaximum and the minimum ones of the measured values is fed back to thesecond exposure apparatus as a correction value if at least one of theabsolute values thereof is determined to be a prescribed value or more.Thus, amounts of displacement can be more effectively reduced. Inaddition to above described steps of the method according to theaforementioned still another aspect of the invention, steps ofsuccessively forming a first layer pattern on a second semiconductorsubstrate then forming a second resist thereon, and a step of formingthe second resist pattern by exposing the second resist using the secondexposure apparatus with a fed back correction value may be provided. Inaddition to above described steps of the method according to the stillanother aspect of the invention, a second resist may be formed again onthe first layer pattern on the first semiconductor substrate, removingthe second resist pattern when a measured value is determined to be aprescribed value or more. Then, a new second resist pattern may beprovided by exposing the new second resist using the second exposureapparatus with a correction value fed back. In this manner, asemiconductor device can be formed utilizing again the semiconductorwafer which has the second resist pattern determined to be defective. Inanother or still another aspect of the invention described above, thefirst exposure apparatus and the second exposure apparatus may be or maynot be the same. Preferably the number of the measuring points ofamounts of displacement is three or more.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing a method of reducing a registration erroraccording to an embodiment of the present invention.

FIG. 2 is a schematic illustration of a result of an exposure by anexposure apparatus to which a correction value is fed back according tothe method of reducing a registration error shown in FIG. 1.

FIG. 3 is a perspective view illustrating a conventional exposureapparatus.

FIG. 4 is a schematic illustration of a result of a measurement ofamounts of displacement (offset amounts).

FIG. 5 is a flow chart showing a conventional method of reducing aregistration error.

FIG. 6 is a schematic illustration of a state of misalignment afterexposures by different exposure apparatuses according to a conventionalmethod.

FIG. 7 is a schematic illustration of a result of an exposure by anexposure apparatus, after correcting the state of misalignment shown inFIG. 6 according to a conventional method for reducing amount ofdisplacement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be hereinafterdescribed with reference to the drawings. FIG. 1 is a flow chart forshowing a method of reducing a registration error according to apreferred embodiment of the invention. FIG. 2 is a schematicillustration of a state of misalignment after an exposure by a secondexposure apparatus with the correction value fed back, shown in FIG. 1.First with reference to FIG. 1, a method of reducing a registrationerror according to this preferred embodiment will be described. Themethod of reducing a registration error in this embodiment shown in FIG.1 is different from the conventional method shown in FIG. 5 in themanner of calculating a correction value.

As shown specifically in FIG. 1, a characteristic of this embodiment isto calculate, in S13, a correction value as one half the sum of themaximum and the minimum values of amounts of displacement at fourpoints. Now, the method of reducing a registration error according tothis embodiment will be described following the flow of FIG. 1.

A first layer is formed on a semiconductor substrate (not shown) in S1.A first resist is then applied to the first layer in S2. In S3, a firstresist pattern shown in S4 is formed by exposing the first resist usinga first exposure apparatus. The first resist pattern is provided suchthat it has a plurality of first misalignment inspection marks.

A first layer pattern in S5 is formed by patterning the lower firstlayer with the first resist pattern as a mask through etching or thelike. The first misalignment inspection marks of the first resistpattern are transferred to the first layer pattern. The first resistpattern is removed after the formation of the first layer pattern. Next,a second layer is formed on the first layer pattern in S6. Second resistis applied to the second layer in S7. In S8, a second resist pattern inS9 is formed by exposing the second resist utilizing a second exposureapparatus. The second resist pattern is provided such that it has aplurality of second misalignment inspection marks corresponding to theplurality of first misalignment inspection marks.

Thereafter, amounts of displacement between the first layer pattern andthe second resist pattern are measured at four points using the firstand the second misalignment inspection marks in S10. In S11, adetermination is made whether at least one of those amounts ofdisplacement is a prescribed value or more. If all amounts ofdisplacement are smaller than the prescribed value, it is determinedthat no correction of exposure apparatus is required, then a secondlayer pattern in S12 is formed by patterning the second layer using thesecond resist pattern as a mask through such as etching. On the otherhand, if at least one of absolute values of the amounts of displacementis determined to be the prescribed value or more in S11, a correctionvalue is calculated in S13, the resist is removed, and then secondresist is formed again in S14.

In a method of calculating a correction value shown in S13 according tothis embodiment, a correction value is calculated as one half the sum ofthe maximum and the minimum values of the amounts of displacement atfour points. The calculated correction value is fed back to the secondexposure apparatus in S8. In S14, the second resist pattern is removedand then new second resist is applied in S7.

By calculating a correction value for the misalignment between the firstlayer pattern 1 and the second resist pattern 2 in FIG. 6 in accordancewith above described calculating method of a correction value, thefollowing result will be provided. If one half the sum of the maximumand the minimum values of the measurements at the four corners in a shotis used as an amount of displacement of the shot shown in FIG. 6, theamount of displacement of the shot is (a+0)×1/2=a/2. The correctionvalue is accordingly a/2, and the result of an exposure by the secondexposure apparatus with the correction value fed back is shown in FIG.2.

Referring to FIG. 2, an amount of displacement of the shot, one half thesum of the maximum and the minimum values of the measurements at fourcorners in the shot is (a/2-a/2)×1/2=0. Further, amounts of displacementat respective measuring points 3 in the shot are -a/2, -a/2, -a/2, +a/2.Thus the maximum absolute value of the amounts of displacement can bereduced to a/2 in this embodiment, while that of the amount ofdisplacement according to the conventional method shown in FIG. 5 isstill 3a/4. It is recognized that the amounts of displacement at fourmeasuring points 3 are averaged. A shift in the Y direction is correctedsimilarly. Thus in this method of the preferred embodiment, a largeamount of displacement at one point in a shot is effectively prevented,since amounts of displacement are distributed over the entire shot. As aresult, such a disadvantage as an electrical short caused due to a shiftof pattern of a contact hole, when, for example, a large amount ofdisplacement occurs at one point in a shot, can be effectively avoided.

It is noted that the flow chart shown in FIG. 1 illustrates a flow inwhich if at least one of the absolute values of the amounts ofdisplacement is determined to be a prescribed value or more in S11, thesecond resist pattern is removed in S14 and second resist is formedagain in S7. That is the flow where the second exposure apparatus isused which has not received the feed back of a correction value. If thesecond exposure apparatus to which a correction value is fed back isused for an exposure, it is not likely that any of the amounts ofdisplacement is determined to be a prescribed value or more in S11, thusa formation of a second layer pattern is proceeded in S12. In this case,the calculation of a correction value in S13 and the removal of thesecond resist pattern in S14 are not executed.

It is also noted that while the amounts of displacement are measured atfour points in this embodiment, a similar effect can be obtained if thenumber of points are three or more according to the invention. Accordingto the preferred embodiment shown in FIG. 1, although the exposure in S3is performed using the first exposure apparatus and the exposure in S8is executed using the second exposure apparatus, a single exposureapparatus may be used for the exposures of S3 and S8.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A method of reducing a registration error in anexposure step during manufacturing a semiconductor device, comprisingthe steps of:calculating one half the sum of maximum and minimum valuesof amounts of displacement measured at a plurality of measuring pointsto obtain a correction value; and using said correction value as acorrection value for an exposure condition setting file within anexposure apparatus employed in said exposure step.
 2. The method ofreducing a registration error according to claim 1, whereinmeasuringpoints of said amounts of displacement is three or more in number.